A conventional technology will be described with reference to FIG. 5 and FIG. 6.
FIG. 5 is a side sectional view of a conventional loudspeaker, and FIG. 6 is a schematic block diagram of a paper-making device of a loudspeaker diaphragm as an essential part of the loudspeaker.
The conventional loudspeaker has magnetic circuit 1, frame 2, conical loudspeaker diaphragm 3, voice coil 4, and damper 5. Magnetic circuit 1 is formed by bonding lower plate 1a having a center pole, annular magnet 1b, and upper plate 1c superimposed on magnet 1b. Magnetic gap 1d is formed between the outer periphery of the center pole and the inner periphery of upper plate 1c. Frame 2 is bonded to upper plate 1c. The outer periphery of loudspeaker diaphragm 3 is bonded to frame 2 via edge 3a, and the lower part of the inner periphery thereof is bonded to voice coil 4 engaged with magnetic gap 1d. Damper 5 for supporting voice coil 4 vertically movably is bonded to voice coil 4 on its inner periphery, and is bonded to frame 2 on its outer periphery.
In the loudspeaker having the above-mentioned configuration, a voice signal is input as an external signal into voice coil 4, thereby moving loudspeaker diaphragm 3 vertically to produce a sound.
The loudspeaker diaphragm is made of paper, resin, or metal foil. Paper having undergone paper-making is generally used as the loudspeaker diaphragm in consideration of the following parameters:                physical properties such as magnitudes of internal loss and rigidity that are essentially required of a loudspeaker diaphragm;        cost; and        good sound making ability with a blend of various wood pulp as materials thereof.        
FIG. 6 shows a producing process of loudspeaker diaphragm 3 formed by the paper-making. The paper-making device shown in FIG. 6 has the following elements: (1) paper-making bath 11 for supplying water having beaten pulp dispersed therein to an after-mentioned paper-making mold, (2) paper-making mold 12 formed of a wire mesh or the like, (3) measuring bath 13 for water having pulp dispersed therein, (4) supply pipe 14, (5) valve 15 for opening and closing a flow channel, (6) drainage pipe 16, and (7) valve 17 for opening and closing a drainage channel.
In a paper-making process using the paper-making device, water where pulp controlled in concentration is dispersed is firstly measured in measuring bath 13, and flow channel opening/closing valve 15 is then opened or closed to supply the pulp to paper-making bath 11 through supply pipe 14.
Thus, the pulp dispersed in a certain amount of water is supplied into paper-making bath 11, and gradually starts to be deposited onto paper-making mold 12 of paper-making bath 11. For performing this process in a short time, generally, the water is rapidly discharged from drainage pipe 16. This process is called as “suki-otoshi” paper-making method. At this time, random vortex occurs near the drainage port in the paper-making bath, and the pulp is deposited on paper-making mold 12 in random stream that can be caused by the random vortex in paper-making bath 11. The deposit is extracted and dried, a center hole is punched, and the outer periphery is removed, thereby providing a loudspeaker diaphragm.
There is another paper-making process in which water having a large amount of pulp dispersed therein is supplied into the paper-making bath, a wire cloth is put into the water having pulp dispersed therein in the paper-making bath, and the wire cloth is raised from the water. This process is called as “suki-age” paper-making method.
The conventional loudspeaker diaphragm and its producing process are disclosed in Japanese Patent Unexamined Publication No. 2003-230197, for example.
The loudspeaker diaphragm employing pulp is inexpensive, allows blending of various pulps, and easily provides a desired acoustic characteristic. However, there are the following difficulties in managing the loudspeaker diaphragm. Random stream in the paper-making bath in the paper-making process causes variation of deposition on pulp paper-making mold 12 and variation of pulp fiber orientation, or large variation of face thickness and face rigidity occurs even on the same circumference in the same diaphragm. As the performance of digital acoustic apparatuses has been recently increased by their development, higher reproducibility has been required of loudspeaker diaphragms.